1. Field of the Invention
This invention relates to an optical wavelength multiplex transmission method which uses a band around a zero dispersion wavelength of an optical fiber and an optical dispersion compensation method for compensating for waveform degradation by a synergetic effect (hereinafter referred to as SPM-GVD effect) of self phase modulation (SPM) and chromatic dispersion (group velocity dispersion: GVD) which is one of several restrictive factors to the transmission distance and the transmission rate in a long-haul, very high-speed optical communication system which employs, for example, an erbium-doped optical fiber amplifier (Erbium-Doped Fiber Amplifier, hereinafter referred to as EDFA).
2. Description of the Related Art
In response to a remarkable increase in the amount of information in recent years, a communication system of a large capacity has become required, and investigations for construction of large capacity communication systems are frequently performed.
For realization of a large capacity communication system, realization by an optical communication system is considered most promising. At present, an optical amplifier multi-repeater system which employs EDFAs is being put into practical use together with, for example, a 2.4 Gb/s optical communication system. In the future, it is forecast that the amount of information will increase progressively as the information-oriented trend advances. It is therefore demanded to build up an optical communication system of an increased capacity corresponding to such increase of the amount of information.
Various methods are available to increase the capacity of an optical communication system, including a TDM (time-division multiplexing) method which involves multiplexing on the time base in order to increase the transmission rate, and a WDM (wavelength-division multiplexing: wherein the wavelength spacing is comparatively great and is called WDM, and wavelength-division multiplexing which involves high concentration multiplexing and is called FDM (frequency-division multiplexing)) method which involves multiplexing on the optical wavelength base.
Of the available methods, a multiplexing method like the TDM method requires an increase of speed of operation of electronic circuits in a transmitter and a receiver in order to increase the transmission rate. At present, several tens Gb/s is considered to be the limit to the speed of operation.
In contrast, with the WDM (FDM) method which makes use of the wide band property of an optical fiber, an increase of capacity to several tens to several hundreds Gb/s is possible by simultaneous exploitation of an increase of the transmission rate, and also the burden to electronic circuits is reduced since multiplexing and demultiplexing are performed simply in an optical region by means of an optical multiplexing apparatus and an optical demultiplexing apparatus (MUX/DEMUX) which employ optical couplers, optical filters and like elements.
In the WDM (FDM) method which involves wavelength multiplexing on the optical frequency base, however, an available band is restricted by gain band dependency of an optical amplifier or wavelength dependency of an optical part. Accordingly, in order to achieve an increase in capacity by multiplexing, the channel spacing must necessarily be decreased to decrease the bandwidth indicated by all channels. Further, in optical transmission of multi-Gigabits, the wavelength of an optical signal must necessarily be set in the proximity of a zero dispersion wavelength of an optical fiber since, otherwise, waveform degradation is caused by chromatic dispersion of the optical fiber.
In an optical communication system to which the WDM (FDM) method is applied in order to achieve such an increase in capacity as described above, however, if the channel spacing is decreased (taking the bandwidth into consideration) and optical signals are set in the proximity of a zero dispersion wavelength of the optical fiber (taking the chromatic dispersion into consideration), an influence of a non-linear effect of the optical fiber, particularly of four wave mixing (hereinafter referred to as FWM), becomes significant, and there is a subject to be solved in that the transmission may be disabled by crosstalk from another channel by such FWM. A similar subject resides in another case wherein wavelength multiplex transmission must be performed in a band in the proximity of the zero dispersion wavelength in order to achieve, for example, upgrading of an existing transmission line.
Meanwhile, as a factor of degradation of the transmission characteristic in the optical amplifier multi-repeater WDM method which particularly makes use of a band in the proximity of a zero dispersion wavelength of an optical fiber, crosstalk by FWM mentioned above is pointed out. The occurrence efficiency of such FWM depends upon the relationship between the zero dispersion wavelength of the optical fiber transmission line and the arrangement of channels.
Three characteristics including: 1. a zero dispersion wavelength, 2. a deviation in zero dispersion wavelength and 3. a dispersion slope (second-order dispersion) are listed as required characteristics for an optical fiber in the WDM method. Those characteristics are closely related to five factors including: a. wavelength multiplexing signal bandwidth, b. gain bandwidth of the EDFA among various optical amplifiers, c. guard band for suppressing FWM (to which the present invention is directed), d. limitation bandwidth by an SPM-GVD effect, and e. presence or absence of an inserted optical dispersion compensator.
By the way, as factors which restrict an increase in distance and an increase in speed of an optical communication system, there are limitations of the loss by an optical fiber and bandwidth limitation by chromatic dispersion. The loss limitation has been almost solved by realization of EDFAs, and it is possible to build up a very long-haul optical communication system for several thousand km or more.
However, the repeater span in a multi-repeater optical amplification system is restricted principally by two factors including: 1. optical SNR (signal to noise ratio) degradation caused by accumulation of ASE (spontaneous emission) from optical amplifier-repeaters, and 2. waveform degradation by an SPM-GVD effect caused by a Kerr effect.
It is already known that, of the two factors, the waveform degradation by an SPD-GVD effect can be compensated for using an optical dispersion compensator having a dispersion value of the opposite positive or negative sign to that of the optical fiber transmission line, and the waveform degradation by an SPM-GVD effect and a dispersion compensation effect can be simulated readily by solving a non-linear Schroedinger equation using the split-step Fourier method.
An optical dispersion compensator used for the object described above is required to cope with a dispersion amount of an optical fiber of a corresponding repeater section and to allow reduction of the number of steps and of the time necessary to realize an optimum dispersion compensation amount and reduction of the cost. Further, the optical dispersion compensation technique is important not only for a 1.55 xcexcm dispersion shifted fiber (hereinafter referred to as DSF) transmission line network being laid at present but also for a long-haul, very high-speed optical communication system and an optical communication system of the WDM (FDM) method which make use of an existing 1.3 xcexcm zero dispersion single mode fiber (hereinafter referred to as SMF) transmission line network.
In a very long-haul optical communication system for several thousand km or more, it is considered desirable to use the zero dispersion wavelength xcex0 of the optical fiber transmission line in order to prevent the dispersion penalty and to use the ordinary dispersion region (dispersion value D less than 0) of the optical fiber in order to minimize the non-linear effect. In order to satisfy the two contradictory requirements, a countermeasure has been proposed which makes use of the ordinary dispersion region for the transmission line and employs an optical dispersion compensator to reduce the apparent dispersion value equal to zero. The optical dispersion compensation technique is effective not only for DSF transmission but also for SMF transmission having a high dispersion value of approximately 18 ps/nm/km.
Various types of optical dispersion compensators have been proposed including dispersion compensating fiber type optical dispersion compensators, transversal filter type optical dispersion compensators and optical resonator type optical dispersion compensators. At present, a dispersion compensating fiber is considered promising from its advantage in that the dispersion compensation amount can be adjusted readily by varying the length of the fiber, and dispersion values higher than xe2x88x92100 ps/(nmxc2x7km) have been obtained by contriving the profile of the core.
The zero dispersion wavelength of an actual optical fiber transmission line presents a deviation in a longitudinal direction. Further, in an optical communication system on land, since it is difficult to set the repeater span to a fixed value (as in a submarine optical communication system), the dispersion amount is not always fixed among different repeater sections. Therefore, ideally an optical dispersion compensator having an optimum dispersion compensation amount is inserted into each repeater section after an actual dispersion amount is measured for the repeater section. However, there is a subject in that such operation requires a great number of steps of operation, long time and a high cost to realize optimum optical dispersion compensators including measurement of dispersion amounts.
It is an object of the present invention to provide an optical wavelength multiplex transmission method wherein, where a band in the proximity of a zero dispersion wavelength of an optical fiber is used, optical signals are disposed at efficient channel spacings taking an influence of the band, the chromatic dispersion, and the FWM into consideration, to realize an optical communication system of an increased capacity which is not influenced by crosstalk by FWM.
It is another object of the present invention to provide an optical wavelength multiplex transmission method wherein the relationship between characteristics required for an optical fiber, particularly, the zero-dispersion wavelength and the deviation in zero-dispersion wavelength, and five specific factors related to the characteristics is made clear to allow establishment of channel arrangement of and transmission line designing for signal light by an optical amplifier multi-repeater WDM method.
It is a further object of the present invention to provide an optical dispersion compensation method by which waveform degradation by an SPM-GVD effect can be compensated for readily without designing or producing optical dispersion compensators suitable for individual transmission lines and dispersion compensation can be performed effectively even when the optical power is not so high that SPM (self phase modulation) does not take place very much, but only waveform degradation is caused by chromatic dispersion (GVD), thereby to reduce the number of steps and the time required to build up an optical communication system and to achieve reduction of the cost.
In order to attain the objects described above, according to an aspect of the present invention, there is provided an optical wavelength multiplex transmission method for multiplexing signal light waves of a plurality of channels having different wavelengths and transmitting the multiplexed signal light using an optical fiber, wherein a four wave mixing suppressing guard band of a predetermined bandwidth including a zero-dispersion wavelength of the optical fiber is set, and the signal light waves of the plurality of channels to be multiplexed are arranged on one of a shorter wavelength side and a longer wavelength side outside the guard band.
In the optical wavelength multiplex transmission method, when signal light waves of a plurality of channels having different wavelengths are multiplexed and transmitted using an optical fiber, since the signal light waves of the plurality of channels to be multiplexed are arranged on one of the shorter wavelength side and the longer wavelength side outside the four wave mixing suppressing guard band of the predetermined bandwidth including the zero-dispersion wavelength of the optical fiber, possible four wave mixing is suppressed. Consequently, an influence from another channel by crosstalk is suppressed.
According to another aspect of the present invention, there is provided an optical wavelength multiplex transmission method for multiplexing signal light waves of a plurality of channels having different wavelengths and transmitting the multiplexed signal light using an optical fiber, wherein a four wave mixing suppressing guard band of a predetermined bandwidth including a zero-dispersion wavelength of the optical fiber is set, and the signal light waves of the plurality of channels to be multiplexed are arranged on the opposite sides of a shorter wavelength side and a longer wavelength side outside the guard band.
In the optical wavelength multiplex transmission method, since signal light waves of a plurality of channels to be multiplexed are arranged on the opposite sides of the shorter wavelength side and the longer wavelength side outside the four wave mixing suppressing guard band, possible four wave mixing is suppressed. Consequently, an influence from another channel by crosstalk is suppressed and efficient utilization of the band can be achieved simultaneously.
The bandwidths of the guard bands may be set in an asymmetrical relationship on the shorter wavelength side and the longer wavelength side with respect to the zero-dispersion wavelength of the optical fiber. In this instance, the channel spacings between adjacent ones of the signal light waves of the plurality of channels may be set different on the shorter wavelength side and the longer wavelength side outside the guard band. Due to the channel spacings thus set, four wave mixing light produced between a signal light wave on the shorter wavelength side and another signal light wave on the longer wavelength side is prevented from coinciding with any of the wavelengths of the signal light waves.
Alternatively, the channel spacings between adjacent ones of the signal light waves of the plurality of channels on each of the shorter wavelength side and the longer wavelength side outside the guard band may be set to an integral number times a constant. Due to the channel spacings thus set, in addition to the advantage that an influence from another channel by crosstalk is suppressed, the channels on the shorter wavelength side and the longer wavelength side outside the guard band can be controlled using Fabry-Perot interferometers of a same characteristic. In this instance, preferably the channel spacings between the channels of the signal light waves of the plurality of channels on the opposite sides of the guard band are set to the integral number times the constant. Due to the channel spacings thus set, the channels on the opposite sides of the shorter wavelength side and the longer wavelength side outside the guard band can be controlled simultaneously using a single Fabry-Perot interferometer of a same characteristic. Or else, the signal light waves of the channels may be arranged such that the signal light waves of no pair or only one pair of ones of the channels have dispersion values which have an equal absolute value. The arrangement further suppresses four wave mixing so that an influence from another channel by crosstalk can be further suppressed.
With the optical wavelength multiplex transmission methods described above, the following effects or advantages can be anticipated.
First, an influence of four wave mixing can be suppressed and the band can be utilized efficiently by arranging signal light waves efficiently. An optical communication system of a large capacity can be realized while maintaining high transmission quality.
Second, even when a zero-dispersion wavelength is positioned within a band of an optical amplifier or within a band of an optical part, signal light waves can be arranged efficiently and compactly while suppressing an effect of four wave mixing within the limited band.
Third, since the channel spacings on the transmission side can be controlled by way of a single or a pair of Fabry-Perot interferometers and an interferometer of the same characteristic to that of the interferometers on the transmission side can be used also on the reception side, control on the transmission side can be simplified and selective reception is facilitated.
According to a further aspect of the present invention, there is provided an optical wavelength multiplex transmission method for multiplexing signal light waves of a plurality of channels having different wavelengths and transmitting the multiplexed signal light using an optical fiber, wherein, taking a zero-dispersion wavelength xcex0 of the optical fiber and a zero-dispersion wavelength deviation range xc2x1xcex94xcex0 of the optical fiber in its longitudinal direction into consideration, the signal light waves of the plurality of channels to be multiplexed are arranged on a shorter wavelength side than a shorter wavelength end xcex0xe2x88x92xcex94xcex0 of the zero-dispersion wavelength deviation range of the optical fiber.
In the optical wavelength multiplex transmission method, when signal light waves of a plurality of channels having different wavelengths are multiplexed and transmitted using an optical fiber, since the signal light waves of the plurality of channels to be multiplexed are arranged on the shorter wavelength side than the shorter wavelength end xcex0xe2x88x92xcex94xcex0 of the zero-dispersion wavelength deviation range of the optical fiber, the zero-dispersion wavelength deviation in the longitudinal direction of the optical fiber is taken into consideration and controlled on the shorter wavelength side of the zero-dispersion wavelength.
A four wave mixing suppressing guard band xcex94xcexg may be provided on the shorter wavelength side than the shorter wavelength end xcex0xe2x88x92xcex94xcex0 of the zero-dispersion wavelength deviation range of the optical fiber, and the signal light waves of the plurality of channels may be arranged on a shorter wavelength side than a wavelength xcex0xe2x88x92xcex94xcex0xe2x88x92xcex94xcexg. In this instance, since the signal light wave of the plurality of channels are arranged on the shorter wavelength side than the wavelength xcex0xe2x88x92xcex94xcex0xe2x88x92xcex94xcexg, taking the four wave mixing suppressing guard band xcex94xcexg into consideration, the zero-dispersion wavelength deviation in the longitudinal direction of the optical fiber is taken into consideration and controlled on the shorter wavelength side of the zero-dispersion wavelength. Thus simultaneously, an influence from another channel by crosstalk is suppressed.
According to a still further aspect of the present invention, there is provided an optical wavelength multiplex transmission method for multiplexing signal light waves of a plurality of channels having different wavelengths and transmitting the multiplexed signal light using an optical fiber, wherein, taking a zero-dispersion wavelength xcex0 of the optical fiber and a zero-dispersion wavelength deviation range xc2x1xcex94xcex0 of the optical fiber in its longitudinal direction into consideration, the signal light waves of the plurality of channels to be multiplexed are arranged on a longer wavelength side than a longer wavelength end xcex0+xcex94xcex0 of the zero-dispersion wavelength deviation range of the optical fiber.
In the optical wavelength multiplex transmission method, when signal light waves of a plurality of channels having different wavelengths are multiplexed and transmitted using an optical fiber, since the signal light waves of the plurality of channels to be multiplexed are arranged on the longer wavelength side than the longer wavelength end xcex0+xcex94xcex0 of the zero-dispersion wavelength deviation range of the optical fiber, the zero-dispersion wavelength deviation in the longitudinal direction of the optical fiber is taken into consideration and controlled on the longer wavelength side of the zero-dispersion wavelength.
A four wave mixing suppressing guard band xcex94xcexg may be provided on the longer wavelength side than the longer wavelength end xcex0+xcex94xcex0 of the zero-dispersion wavelength deviation range of the optical fiber, and the signal light waves of the plurality of channels may be arranged on a longer wavelength side than a wavelength xcex0+xcex94xcex0+xcex94xcexg. Due to the provision of the four wave mixing suppressing guard band xcex94xcexg and the arrangement of the signal light waves, the zero-dispersion wavelength deviation in the longitudinal direction of the optical fiber is taken into consideration and controlled on the longer wavelength side of the zero-dispersion wavelength, and simultaneously, an influence of another channel by crosstalk is suppressed.
The signal light waves of the plurality of channels may be arranged within a transmissible band defined by an allowable dispersion value determined from a synergetic effect of self phase modulation and group velocity dispersion in the optical fiber. Where the signal light waves are arranged in this manner, they can be arranged taking wavelength degradation by an SPM-GVD effect into consideration. Further, although SPM does not take place very much and only waveform degradation by chromatic dispersion (GVD) occurs when the optical power is not very high, the signal light arrangement can be performed also taking such waveform degradation into consideration.
The signal light waves of the plurality of channels may be arranged outside the transmissible band defined by the allowable dispersion value determined from the synergetic effect of self phase modulation and group velocity dispersion in the optical fiber, and the zero dispersion wavelength xcex0 of the optical fiber may be apparently shifted using an optical dispersion compensator to apparently arrange the signal light waves of the plurality of channels into the transmissible band. Due to the arrangement of the signal light waves and the shift of the zero dispersion wavelength xcex0, the signal light waves can be arranged taking waveform degradation by an SPM-GVD effect into consideration.
The optical wavelength multiplex transmission method may be constructed such that, taking a dispersion compensation amount deviation range xc2x1xcex4xcexDC of the optical dispersion compensator into consideration, a band xcex94xcexWDM within which the signal light waves of the plurality of channels are to be arranged is set expanding the same by the dispersion compensation amount deviation range xcex4xcexDC on the opposite sides of the longer wavelength side and the shorter wavelength side. Due to the band xcex94xcexWDM thus set, the signal light waves can be arranged taking the dispersion compensation amount deviation of the optical dispersion compensator into consideration.
The signal light waves of the plurality of channels may be arranged in a gain band of an optical amplifier connected to the optical fiber. Due to the arrangement of the signal light waves, the powers of the signal light waves can be made equal to each other and also the receive characteristics of the signal light waves can be made equal to each other.
A band xcex94xcexWDM within which the signal light waves of the plurality of channels are to be arranged may be set expanding the same in accordance with optical wavelength variations of the signal light waves of the plurality of channels. Due to the band xcex94xcexWDM thus set, the productivity of light sources of the signal light waves and the variation of each signal light wave by the wavelength control accuracy are taken into consideration.
With the optical wavelength multiplex transmission methods described above, the following effects or advantages can be anticipated.
First, in a wavelength division multiplexing method which makes use of a band in the proximity of the zero-dispersion wavelength xcex0 of the optical fiber, the signal light waves of the individual channels can be arranged without being influenced by four wave mixing, and simultaneously, required characteristics regarding the zero-dispersion wavelength xcex0 for an optical fiber transmission line to be laid can be made clear. Consequently, channel arrangement of and transmission line designing for signal light by an optical amplifier multi-repeater WDM method can be established.
Second, the zero-dispersion wavelength deviation in the longitudinal direction of the optical fiber is taken into consideration and controlled, and simultaneously, an influence of four wave mixing is suppressed so that an influence from another channel by crosstalk is suppressed. Consequently, a high degree of transmission accuracy can be maintained.
Third, signal light waves can be arranged taking waveform degradation by an SPM-GVD effect into consideration, and where the signal light waves of different channels are arranged in the gain bandwidth xcex94xcexEDFA of the EDFA, the powers of the signal light waves can be made equal to each other and the receive characteristics of the signal light waves can be made equal to each other.
Fourth, where a signal light band is set expanding the same in accordance with optical wavelength variations of the signal light waves of the channels, the variations of the signal light waves arising from the productivity and/or the wavelength control accuracy of light sources of the signal light waves such as semiconductor lasers are taken into consideration, and where an optical dispersion compensator is employed, by setting the signal light band expanding the same by a dispersion compensation amount deviation range on the opposite sides of the shorter wavelength side and the longer wavelength side, also the dispersion compensation amount deviation of the optical dispersion compensator is taken into consideration. Consequently, optical transmission of higher reliability can be achieved.
According to a yet further aspect of the present invention, there is provided an optical dispersion compensation method for compensating for a dispersion amount of an optical transmission system which includes a transmitter, a repeater and a receiver and transmits signal light from the transmitter to the receiver by way of the repeater, comprising the steps of preparing in advance two kinds of optical dispersion compensator units having dispersion amounts having different positive and negative signs, inserting the two kinds of optical dispersion compensator units separately into the optical transmission system, and selecting one of the two kinds of optical dispersion compensator units which provides a better transmission characteristic to the optical transmission system and incorporating the selected optical dispersion compensator unit into the optical transmission system.
In the optical dispersion compensation method, since two kinds of optical dispersion compensator units having dispersion amounts having different positive and negative signs are prepared in advance and inserted separately into an optical transmission system to select one of the two kinds of optical dispersion compensator units which provides a better transmission characteristic to the optical transmission system, the dispersion amount of the optical transmission system can be compensated for simply when an accurate dispersion amount cannot be measured but the zero-dispersion wavelength deviation can be grasped to some degree.
According to a yet further aspect of the present invention, there is provided an optical dispersion compensation method for compensating for a dispersion amount of an optical transmission system which includes a transmitter, a repeater and a receiver and transmits signal light from the transmitter to the receiver by way of the repeater, comprising the steps of preparing in advance two kinds of optical dispersion compensator units having dispersion amounts having different positive and negative signs, measuring a dispersion amount of the optical transmission system, and selecting one of the two kinds of optical dispersion compensator units which has a dispersion amount whose sign is opposite to that of a measured dispersion amount and incorporating the selected optical dispersion compensator unit into the optical transmission system.
In the optical dispersion compensation method, since two kinds of optical dispersion compensator units having dispersion amounts having different positive and negative signs are prepared in advance and, when the dispersion amount of an optical transmission system can be measured, the dispersion amount is measured and then one of the two kinds of optical dispersion compensator units which has a dispersion value whose sign is opposite to that of a thus measured dispersion value is selected, the dispersion amount of the optical transmission system can be compensated for further reliably.
According to a yet further aspect of the present invention, there is provided an optical dispersion compensation method for compensating for a dispersion amount of an optical transmission system which includes a transmitter, a repeater and a receiver and transmits signal light from the transmitter to the receiver by way of the repeater, comprising the steps of preparing in advance a plurality of kinds of optical dispersion compensator units having different dispersion amounts having different positive and negative signs, selectively inserting the plurality of kinds of optical dispersion compensator units into the optical transmission system changing the installation number and the combination of the optical dispersion compensator units, and selecting an installation number and a combination of the optical dispersion compensator units from within the plurality of kinds of optical dispersion compensator units which provide a good transmission characteristic to the optical transmission system and incorporating the optical dispersion compensator units of the selected installation number and combination into the optical transmission system.
In the optical dispersion compensation method, since a plurality of kinds of optical dispersion compensator units having different dispersion amounts having different positive and negative signs are prepared in advance and selectively inserted into an optical transmission system changing the installation number and the combination of the optical dispersion compensator units and then an installation number and a combination of the optical dispersion compensator units which provide a good transmission characteristic to the optical transmission system are selected from within the plurality of kinds of optical dispersion compensator units, the dispersion amount of the optical transmission system can be compensated for simply and optimally when the zero-dispersion wavelength deviation is unknown or the zero-dispersion wavelength and the wavelengths of the signal light waves are displaced by great amounts from each other.
According to a yet further aspect of the present invention, there is provided an optical dispersion compensation method for compensating for a dispersion amount of an optical transmission system which includes a transmitter, a repeater and a receiver and transmits signal light from the transmitter to the receiver by way of the repeater, comprising the steps of preparing in advance a plurality of kinds of optical dispersion compensator units having different dispersion amounts having different positive and negative signs, measuring a dispersion amount of the optical transmission system, and selecting an installation number and a combination of the optical dispersion compensator units from within the plurality of kinds of optical dispersion compensator units, with which dispersion values of the signal light waves fall within a transmissible dispersion value range, in accordance with a measured dispersion value and incorporating the optical dispersion compensator units of the selected installation number and combination into the optical transmission system.
In the optical dispersion compensation method, since a plurality of kinds of optical dispersion compensator units having different dispersion amounts having different positive and negative signs are prepared in advance and, when the dispersion amount of an optical transmission system can be measured, the dispersion amount is measured and then an optimum installation number and an optimum combination of such optical dispersion compensator units are selected in accordance with a thus measured dispersion amount, the dispersion amount of the optical transmission system can be compensated for so that it may fall within an allowable dispersion value range with certainty.
The optical dispersion compensator units may be additionally incorporated into at least one of the transmitter, the repeater and the receiver of the optical transmission system to incorporate the optical dispersion compensator units into the optical transmission system.
When the optical transmission system performs optical wavelength multiplex transmission to multiplex and transmit signal light waves of a plurality of channels having different wavelengths, the signal light waves may be demultiplexed for each one wave by wavelength demultiplexing and the optical dispersion compensator units may be provided for the individual channels of the signal light waves of the wavelengths in the optical transmission system, or the signal light waves may be demultiplexed for each plurality of waves and the optical dispersion compensator units may be provided for the individual channel groups each including a plurality of signal light waves in the optical transmission system, or else the optical dispersion compensator units may be provided for all of the signal light waves of the plurality of channels in the optical transmission system.
Each of the optical dispersion compensator units may be additionally provided with an optical amplifier for compensating for an optical loss of the optical dispersion compensator unit. Due to the additional provision of the optical amplifier, the optical loss of each optical dispersion compensator unit can be compensated for. In this instance, a pair of optical amplifiers may be additionally provided at a preceding stage and a next stage to each of the optical dispersion compensator units. Due to the additional provision of the optical amplifiers, the noise figure (hereinafter referred to as simply NF) of the optical amplifier at the preceding stage can be set low.
The optical dispersion compensator units may be constructed as a package wherein they are mounted on a circuit board so that the optical dispersion compensator units may be replaced or incorporated in units of a package. Due to the construction of the optical dispersion compensator units, the dispersion compensation amount can be varied readily.
According to a yet further aspect of the present invention, there is provided an optical dispersion compensation method for compensating for a dispersion amount of an optical transmission system which includes a transmitter, a repeater and a receiver and transmits signal light from the transmitter to the receiver by way of the repeater, comprising the steps of incorporating, in advance into at least one of the transmitter, the repeater and the receiver of the optical transmission system, a plurality of kinds of optical dispersion compensator units having different dispersion amounts having different positive and negative signs in such a connected condition as to allow switching of a selective combination of the optical dispersion compensator units by means of switching means, and operating the switching means to select a suitable combination of the optical dispersion compensator units from within the plurality of types of optical dispersion compensator units and incorporating the optical dispersion compensator units of the selected combination into the optical transmission system.
In the optical dispersion compensation method, since a plurality of kinds of optical dispersion compensator units having different dispersion amounts having different positive and negative signs are incorporated in advance in at least one of a transmitter, a repeater and a receiver of an optical transmission system in such a connected condition as to allow switching of a selective combination of the optical dispersion compensator units by means of switching means, a suitable combination of the optical dispersion compensator units can be selected from within the plurality of types of optical dispersion compensator units.
The switching means may be operated in response to a control signal from the outside. In this instance, the optical dispersion compensation method may be constructed such that the switching means is operated in response to a control signal from the receiver to switch the combination of the optical dispersion compensator units while a transmission characteristic of the optical transmission system is measured simultaneously by the receiver to determine a combination of the optical dispersion compensator units which provides an optimum transmission characteristic to the optical transmission system, and the switching means is operated in response to another control signal from the receiver to switch the combination of the optical dispersion compensator units to the determined combination which provides the optimum transmission characteristic to the optical transmission system. The switching means may include a mechanical switch or an optical switch.
With the optical dispersion compensation methods described above, the following effect or advantage can be achieved. In particular, waveform deterioration by an SPM-GVD effect and/or the dispersion amount of a guard band can be compensated for readily without designing or producing optical dispersion compensators suitable for individual transmission lines, and reduction of the number of steps and the time required to build up an optical communication system can be realized.
Further objects, features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings in which like parts or elements are denoted by like reference characters.